Insect identification > Homoptera > Scale insects > San Jose scale

San Jose scale

The San Jose scale (Aspidiotus perniciosus Comst.). - This is one of the most serious pests among the scale insects. Its original home was probably China, but it appears to have reached California about 1870 and since then has spread practically all over this country. It has a wide range of food plants, on many of which it thrives sufficiently to kill them quickly. The plants which suffer most from its attacks are the fruit trees and currants, the dog-woods, thorns, poplars, ornamental cherries and plums, hardy roses, willows, lilacs and lindens; even maples and elms are sometimes attacked, the total list of plants upon which it has been found numbering over a hundred. It feeds on all parts of the plant above ground, even including the fruit.

The full-grown female scale is about the size of a pin head, nearly circular in outline and rather flat, sloping gradually upward from its edge to near the center, where a slight circular depression surrounds the raised center or "nipple" itself. It is brownish gray in color when adult but in earlier stages may vary from this. The adult male scale is somewhat smaller, more oval in outline, and with the nipple not centrally placed but nearer one end.

At the beginning of the winter season specimens of this scale of practically all ages occur, but probably only those from about one-third to one-half or two-thirds grown survive the winter. In the spring these individuals resume their feeding on the sap and after a time the males appear. In the Northern states this condition is hardly reached before the middle of May, but at Washington, D.C., it comes early in April, and farther south still earlier.

After mating, the females continue to grow and about a month later the first young appear. These do not, in the San Jose scale, hatch from eggs laid by the parent but the young are born alive; i.e., this insect is ovoviviparous. These young are produced, a few every day or two, and the parent lives for a month or more, producing an average total of about 400 young. These resemble the crawling young of the scales already considered, except that they are lemon yellow in color, and they crawl about and settle down to feed in the same way.

The scale now begins to appear, at first as white, waxy threads over the back, which soon mat together to form a pure white covering. As the nymph beneath molts, the molted skins are added to this and variations in color of the scale appear.

Sometimes the scale of the partly grown insect may show white, black and gray, varying in arrangement according to the completeness with which the different parts have combined, but before maturity it becomes a quite uniform brownish-gray.

The young become adult in a little over a month and then themselves begin to produce young, and in the Northern states there are usually at least three generations in a season, while in the South there are four or even more. The generations overlap, the earliest young produced by the second generation, for example, sometimes appearing before the last born of the preceding one, which results in the almost constant presence of crawling young on an infested tree, from the time the first one appears until reproduction is stopped by cold weather.

Assuming the production of four full generations in a season, equally divided between the sexes, and with no loss in number from death by accident or other causes to reduce the number produced, we have a total of 3,216,080,400 individuals as the descendants during one season from a single pair. Fortunately, many never reach maturity, or an infested tree would often be sucked dry before winter.

The San Jose scale has a number of parasites which are sometimes quite effective, destroying a large percentage of the scales in some localities; but with such an enormous power of increase of the pest, even a high degree of parasitism fails to give the relief needed.

A few predaceous insects are also known which feed upon the scale. Most noticeable among these is the twice-stabbed ladybeetle (Chilocorus bivulnerus Muls.), a small black beetle with two red spots. It is nearly circular in outline, very convex and is about one-eighth of an inch long. A fungous disease also attacks the scale, particularly in the South, but parasites, predaceous foes and diseases together generally fail to hold it entirely in check.

A ladybeetle closely resembling the twice­stabbed ladybeetle is an enemy of the scale in China, the native home of the pest, and this insect has been brought to the United States with the hope that it might do effective work here, but thusfar, for various reasons, it has failed to accomplish much.